In general, offshore passive heave compensators are designed to reduce the impact of significant load variations that are possible during offshore heavy lifting or transfer, generally due to the variable motions of either the lifting point or the load, or indeed both. Such compensators can reduce the dynamic forces in the lifting wire or host wire caused by the motion of the sea from or on the load, to the lifting point. They use a mixture of hydraulics and pneumatic dampers to help compensate for such load variations. Their uses include the transfer or lifting of loads: between static points and floating vessels, between floating vessels, or offloading units from such vessels; as well as in various subsea operations such as lowering units to a sea bed, and in ‘overload protection’ in subsea environments.
U.S. Pat. No. 7,934,561 B2 discloses a subsea passive heave compensator as an ‘in line tool’ that uses the principles of spring isolation to generate a net heave compensation effect or spring isolation effect. The unit is a ‘nitrogen over oil’ spring dampening device, and its invention uses ‘pressure balancing’ to mitigate/eliminate the effect that hydrostatic pressure has on such units when used in deeper water.
Ernst-B. Johansen a.s. supply units under the trade mark ‘Cranemaster’ (www.cranemaster.no) as passive heave compensating systems, which are self-contained hydraulic/pneumatic units charged with an internal gas pressure and oil volume. The Cranemaster units are able to absorb peak loads: such as for example a transfer lift between a supply vessel and a rig, or at splash zones, when items with large surface areas are lowered through the splash zone.
The Cranemaster units generally have a different preset “point of stroke” (i.e. the load at which the piston will start to extend, achieved by adjusting the pretension pressures etc.), depending on their use. For example, where a Cranemaster unit is used in a splash zone or for subsea lifts, the Cranemaster is normally pre-set so that the piston rod initially extends to about ⅔ of its length when in air, and retracts to about ⅓ of its length when submerged. Above water, the Cranemaster unit is free cycle, responding to its surroundings and catching slack wire when being in line to lift a load through a splash zone. When the load is fully submerged, the unit then acts as a passive heave compensator to reduce the vertical displacement and thus landing speed of the load. When working mainly as a subsea heave compensator, the unit is instead pre-set to cycle about ⅓ to ½ of its full stroke. This leaves the load in a more stable state as the unit absorbs motion and vibrations transmitted by the wire. As the load being installed will be exposed to less dynamic motion, it is possible to control the vertical displacement and landing speed, and thereby provide a safer installation.
For transfer lifts between vessels, the unit can be preset to absorb peak loads due to the vessel's motion in the waves relative to a rig.
The Cranemaster unit can also be pre-set to reduce or avoid resonance.
For retrieving objects from the sea bed, the Cranemaster unit can be pre-set to initially be at a zero stroke. Thus, when used for retrieving subsea structures, the unit is able to take up the vessel's motion ‘topside’ on the surface, and protect the crane from overload. The unit uses its spring force provided by the pre-set gas pressure to ensure a controlled retrieval of the subsea equipment from its installed position. As the crane starts lifting, the increased tension in the crane wire makes the unit extend until the tension of the crane wire exceeds the force necessary for pulling the object out of the seabed. Once lifted, the unit can ensure a controlled retraction, catching up slack wire and acting as a passive heave compensator as the load is lifted towards the surface.
However, the Cranemaster units cannot be changed from their pre-set compensation during operation, and they are only efficient when operating in the conditions that they were tuned for. They are distinctly less effective when used for different uses, and they cannot be pressure-compensated for use at different depths. This becomes a more significant issue at very deep depths.
FIG. 1 of the accompanying drawings shows a cross-sectional schematic view of an example of a prior art passive heave compensator 1 to illustrate this. The prior art compensator 1 has a main hydraulic cylinder 2 housing a moveable piston 4 and piston rod 6. The compensator 1 is generally in line with the lifting of a load, and thus has a ‘top’ and ‘bottom’ relative to the vessel or seabed etc. The top of the main hydraulic cylinder 2 has an upper connection point (not shown) to attach to a lifting wire or the like, and a load (not shown) is attached to the free end of the piston rod 6 using a lower connection point (not shown). The piston 4 has a piston head 8 which completely divides the main hydraulic cylinder 2 between a first compressed gas portion 10, and a first oil portion 12. Next to the main hydraulic cylinder 2 is an accumulator 14 having a moveable separator 16 dividing the accumulator volume into a second gas portion 18, and a second oil portion 20. The first and second oil portions 12, 20 are in communication via a passageway 22.
In use, the expected load and the loading limits for the intended use of the passive heave compensator are considered, and the volume and pressure of each of the gas and oil portions are calculated so as to provide the correct range of damping to the expected load or loading, based on an expected or desired displacement of the piston within the main hydraulic cylinder. In this way, the passive heave compensator is ‘pre-tensioned’ to the expected load variation or other load limitations.
However, as the passive heave compensator shown in FIG. 1 is a self-contained unit, the pressures and volumes of the gas and oil phases cannot be adjusted during operation. That is, the compensator is considered to be ‘pre-set’. As such, a prior art compensator is not efficient when used in any different load conditions, or for significantly different loads, or even for different uses.